1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method. More particularly, the present invention relates to an image processing apparatus and an image processing method that are used for processing an image of which each pixel consists of one color signal and generating an image of which each pixel at the same position consists of plurality of color signals.
2. Description of the Related Art
Various types of electronic image capturing apparatuses such as a digital camera or a digital video camera that captures an image of a subject have conventionally appeared. The configuration of an imaging device that is built in an electronic image capturing apparatus is roughly classified into the configuration of a three-plate imaging device that can obtain a color signal of three color components of red (R), green (G), and blue (B) at one pixel position and the configuration of a single-plate imaging device that can obtain a color signal of only one color component of three color components at one pixel position. For example, a currently available digital camera generally has the configuration of a single-plate imaging device, and a digital video camera has the configuration of a three-plate imaging device and also has the configuration of a single-plate imaging device.
A three-plate imaging device is expensive because its configuration is complicated and the number of parts is large. However, the three-plate imaging device is generally high picture quality because a color signal of three color components of R, G, and B can be obtained at a capturing point at each pixel position of a captured image.
On the other hand, a single-plate imaging device has a simple configuration. However, each filter of R, G, and B should be arranged on the front of an imaging device in a mosaic shape every pixel in order to obtain a color signal of three color components of R, G, and B, and only a single-component color signal can be obtained at each pixel position of a captured image.
As a mosaic-like arrangement of each filter of R, G, and B, it is general that a unit filter group is formed as four filters by arranging an R filter and a B filter at a diagonal position and arranging a pair of G filters at a diagonal position and unit filter groups are arranged as a two-dimensional array (hereinafter, “Bayer array”).
In the configuration of a single-plate imaging device that has the Bayer array, a color signal of three color components per a pixel is obtained by performing an interpolation process on a missing color signal at each pixel position by using a color signal at an adjacent pixel position. Such a process is generally referred to as a demosaic process.
The demosaic process requires performing an interpolation process that is suitable for the edge structure of a target image at a missing pixel position of a signal of a specified color. When a simple interpolation process that does not consider the edge structure of a target image is performed in the demosaic process, there occurs a phenomenon called a false color indicating that a color that originally does not exist appears in a pixel area such as a peripheral portion of an edge.
Moreover, a signal of a specified color is a color signal that has a specified color component among plurality of color signals that forms an image. For example, a color signal of a G component is illustrated in an image formed of three color signals of R, G, and B. Moreover, a missing pixel position is position of a pixel, which does not have a signal of specified color, among pixels that form an image.
For example, in the case of an image formed of three color signals of R, G, and B, sampling intervals of color signals of an R component and a B component to a color signal of a G component (an example of a signal of a specified color) are different in a pixel area such as an edge having a high spatial frequency. For this reason, when a simple interpolation process is performed in the demosaic process, a high spatial frequency that can be sufficiently expressed by the color signal of a G component is turned down toward a low frequency side in each color signal of an R component and a B component, and thus a false color occurs.
As a method for reducing such a false color, there is a method for forming, on an imaging device, an image that is obtained by degrading a spatial frequency characteristic of an optical system such as a lens or an optical low pass filter up to a spatial frequency at which a sampling interval of a color signal of an R component or a B component of a Bayer array can be reproduced. At this time, the reduction of resolution of an image that is formed on the imaging device is generally suppressed by the design of the optical system. For example, in order to suppress the reduction of resolution of an image, the optical system is designed in such a manner that a spatial frequency is a frequency at which a moire caused by turndown distortion does not occur in the sampling interval of the color signal of a G component. Moreover, a conventional technique for performing the suppression of the reduction of resolution of an image and the reduction of a false color by the design of the optical system has been known as disclosed in, for example, Japanese Laid-open Patent Publication No. 2008-35470.
On the other hand, there is an interpolation processing apparatus disclosed in Japanese Laid-open Patent Publication No. 2000-224602 as a conventional technique for performing the reduction of a false color of an image. In the interpolation processing apparatus disclosed in the document, an imaging device having a Bayer array computes a similarity degree of a first direction and a similarity degree of a second direction at a vacant lattice point that is a position (hereinafter, “G-signal missing position”) at which the color signal of a G component is missed and determines a direction having a strong similarity. Next, the interpolation processing apparatus detects a vacant lattice point at which a predetermined association does not exist in a direction having a strong similarity between vacant lattice points adjacent to each other, on the basis of the determination result. After that, in accordance with a method different from the determination, the interpolation processing apparatus computes a similarity degree of the first direction and a similarity degree of the second direction to again determine a direction having a strong similarity and computes an amount of interpolation in accordance with the modified determination result.